Oouf, Ballerinacurls your situation is a tough one. From what you stated, it sounds to me that the mom's reaction has more to do with her own insecurities than how your hair really looks. So when she criticizes your hair, I'm sure it's because she's insecure about her own. She must wish that she had straight, thick hair, but she can't achieve it. So she transfers those desires and insecurities on to you. People use the words that would hurt them the most when they attack others. So probably your mom dreads someone telling her that her own hair is messy and unkempt.
And now that you're not conforming to her ideals of beauty, she'll try to bully you into doing so. But you have to remember that it's your hair, not hers. God gave her a head of hair, and she fried it. She doesn't get to have a second round with yours! If she hadn't tried so hard to conform to a standard of beauty that wasn't meant for her, then maybe she wouldn't be so unhappy about her own hair.